Great info from weightlossforall.com

CALCULATE TARGET HEART RATE ZONE
To perform aerobics effectively you need to calculate your target heart rate zone. To do this subtract your age from 220 to find out your maximum heart rate, so if a person is 30 years old then:

Maximum heart rate.. 220 - 30 = 190 beats per minute

Then multiply your maximum heart rate by 65%:

190 x 65% = 123.5 beats per minute

123 beats per minute will be the lower range of the zone.

Now work out your higher range by multiplying your maximum heart rate by 85%:

190 x 85% = 161.5 beats per minute

The example reveals a target heart rate of 123 - 161. When exercising the heart rate should be within your own range at all times and to burn more fat you should exercise within the lower range. Exercising at the lower range enables the body to take up enough oxygen so the cells can utilize stored fat. If you workout within the higher range of the zone you will burn more calories however, for most overweight people, the majority of energy used will be in the form of carbohydrates and less used up from fat storage.

You can check your heart rate during a workout, but first slow down the pace a little so you can concentrate more. Gently place your index and middle finger on the inner part of your wrist. Now count how many beats in 10 seconds, then multiply the number by 6. I would recommend using an automatic heart rate monitor so you can concentrate more on exercising effectively to burn more fat.